The present invention relates to a stage apparatus for positioning a workpiece or other object to be processed or machined. More particularly, the present invention relates to a stage apparatus suitable for use in an exposure system.
In an exposure system used to produce, for example, semiconductor devices, liquid crystal display devices, image pick-up devices (e.g. CCDs), or thin-film magnetic heads, a reticle stage or a wafer stage is used as a stage apparatus for positioning a reticle (or a photomask, etc.) or a wafer (or a glass plate, etc.). These days, exposure systems of the scanning exposure type, e.g. step-and-scan exposure systems, are also used in which a pattern on a reticle is sequentially transferred onto a wafer by synchronously scanning the reticle and the wafer relative to a projection optical system. Stage apparatuses used in such scanning exposure type systems are also demanded to have the function of scanning the reticle and the wafer at a constant speed and with high accuracy.
Reticle and wafer stages for an exposure system are each provided with a laser interferometer (i.e. laser light wave interference type length measuring device) for precisely measuring the position of the stage at the time of positioning or scanning. The laser interferometer measures the position of the stage by applying a laser beam to a moving mirror secured to the stage and receiving the laser beam reflected from the moving mirror. In this case, if there are fluctuations of air in the optical path of the laser beam, an error is introduced into the measured value by the laser interferometer. Such fluctuations of air are, in many cases, caused by a turbulent flow or air or a change in the environmental temperature in the vicinity of the optical path. Therefore, there has been a demand for a method of minimizing a turbulent flow of air and a temperature change in the vicinity of the optical path.
Recently, linear motors have been used in reticle and wafer stages as driving mechanisms for driving the stages at high speed and in a non-contact fashion. A typical linear motor consists essentially of a stationary member secured to a base member and a moving member secured to a member that moves relative to the base member. When the stationary member includes a coil, the moving member includes a magnetic field-generating member, e.g. a magnet, whereas, when the stationary member includes a magnetic field-generating member, the moving member includes a coil. A linear motor of the type wherein a magnetic field-generating member is included in the moving member and a coil is included in the stationary side is called "moving magnet type linear motor". A linear motor of the type wherein a coil is included in the moving member and a magnetic field-generating member is included in the stationary member is called "moving coil type linear motor".
Both the moving magnet type linear motor and the moving coil type linear motor structurally need a wide gap between the coil and the magnetic field-generating member in comparison to the ordinary rotary motors and therefore tend to exhibit a lower efficiency and to generate a larger amount of heat than the ordinary rotary motors.
In many cases, driving coils used in linear motors according to the foregoing prior art have a structure in which the coils are exposed in the air. Accordingly, heat from the coils causes a change in the ambient air temperature. This causes fluctuations of air around the optical path of a laser beam emitted from a laser interferometer that measures the position of the reticle or wafer stage, resulting in errors in the measured values by the laser interferometer. However, the errors have heretofore been within a specified tolerance in most cases. These days, however, the integration degree of semiconductor devices and the like is increasing more and more, and a correspondingly high positioning accuracy is required for exposure systems. Therefore, it is demanded to reduce measuring errors due to local fluctuations of air and other disturbance.
Accordingly, there has recently been proposed a method wherein the area around coils that generate a large amount of heat is covered with a cylindrical container, and a cooling fluid is passed through the container by a temperature controller to prevent a rise in temperature which would otherwise be caused by heat generated from the coils. This method makes it possible to suppress a rise in temperature of the heat-generating portion. However, according to this method, a cooling fluid is simply passed through the heat-generating portion. That is, a cooling fluid is passed in disregard of the position of a laser interferometer, or in a case where a plurality of linear motors are installed, the linear motors are cooled successively by a cooling fluid through a single system of circulating piping. Therefore, the conventional method cannot attain the object to eliminate a change in the ambient temperature of the optical path of the laser beam from the laser interferometer to thereby surely control local fluctuations of air around the optical path.
In view of the above-described circumstances, a first object of the present invention is to provide a stage apparatus that uses a linear motor to drive a movable portion and that measures the position of the movable portion with an interferometer. The stage apparatus is designed to suppress fluctuations of air in the optical path of a light beam emitted from the interferometer due to heat generated from the linear motor, thereby enabling the position of a stage to be measured with high accuracy, and thus allowing the stage to be accurately positioned or moved.
In an exposure system, e.g. stepper, a wafer stage, for example, is used as a device for moving a wafer to a predetermined exposure position. The wafer stage includes an X-axis stage and a Y-axis stage, which are movable in respective directions parallel to X- and Y-axes perpendicularly intersecting each other. These days, particularly, a hydrostatic air guide type stage apparatus that uses a hydrostatic gas bearing is used as a wafer stage for realizing high-speed and high-accuracy positioning. Regarding exposure systems, attention has also been paid to scanning exposure type projection exposure systems such as step-and-scan type exposure systems, in which exposure is carried out by synchronously scanning a reticle and a wafer. In such a scanning exposure type projection exposure system, a hydrostatic air guide type stage apparatus is used not only for a wafer stage but also for a reticle stage. Further, hydrostatic air guide type stage apparatuses are also used for positioning workpieces or other objects to be processed or machined, for example, in precision machine tools or precision measuring machines.
A conventional hydrostatic gas bearing used in such a stage apparatus is schemed to maintain a constant gap between a movable portion and a stationary portion by keeping a constant static pressure between the movable portion and the stationary portion, thereby enabling the movable portion to be moved smoothly at high speed. The conventional hydrostatic gas bearing has a hydrostatic gas outlet and inlet provided in the bearing surface of the movable portion or the stationary portion (there are cases where no inlet is provided). Thus, a constant gap is maintained between the bearing surface and a guide surface facing the bearing surface by the balance of repulsion force produced by blowoff of compressed air supplied from the outside and suction force (or gravity).
In an exposure system, laser interferometers are used to measure the positions of a reticle stage and wafer stage. A laser interferometer applies a laser beam to a moving mirror secured to a movable portion to measure an amount of displacement of the movable portion. In this case, if there are fluctuations of air in the optical path of the laser beam, an error is introduced into the measured value by the laser interferometer. Therefore, measures have heretofore been taken to suppress the fluctuations of air in the optical path of the laser beam. For example, temperature-controlled air is supplied to the surroundings of the optical path of the laser beam.
A conventional stage apparatus using a hydrostatic gas bearing has a structure in which the compressed air blown is constantly released as it is to the periphery of the air outlet provided in the bearing surface. However, in general factories, the temperature of compressed air or the like that is supplied to the hydrostatic gas bearing is not satisfactorily controlled. Even if the compressed air is at room temperatures, when it is blown out from the air outlet provided in the bearing surface, the air pressure reduces to the atmospheric pressure, and the temperature of the air is undesirably lowered by adiabatic cooling. Accordingly, the ambient temperature of the workpiece lowers below the desired temperature. Consequently, it is likely that a positioning error or a machining error will occur on account of the contraction of the workpiece, the stage, etc. As has been stated above, many of stages for exposure systems use laser interferometers that use laser beams to measure coordinate positions. Accordingly, it is necessary in order to perform accurate coordinate measurement to suppress a turbulent flow of air in the optical path of a laser beam emitted from such a laser interferometer and a temperature change in the optical path. However, if air different in temperature from the environmental air is discharged from a hydrostatic gas bearing, the air stream in the optical path of the laser beam is made turbulent. This causes temperature variations in the optical path, resulting in a degradation of the measuring accuracy of the laser interferometer.
Compressed air to be supplied to a hydrostatic gas bearing is not satisfactorily cleaned in general factories. Even air that is cleaned to a high degree cannot avoid mixing of fine foreign substances. Therefore, the workpiece may be contaminated by fine foreign substances or chemical substances released into the air from the hydrostatic gas bearing. It is necessary to prevent mixing of such foreign substances particularly in factories where a high level of cleanliness is required to produce semiconductor substrates, liquid crystal substrates, etc.
In view of the above-described circumstances, an object of the present invention is to provide a stage apparatus designed so that when a hydrostatic gas bearing is used, the amount of air released irregularly to the surroundings of a bearing surface is minimized to suppress contamination on a stage and to minimize a change in the environmental temperature. A second object of the present invention is to provide a stage apparatus designed so that when an interferometer is used in combination with a hydrostatic gas bearing, the measuring accuracy of the interferometer is improved to accurately measure the position of a stage, thereby enabling the stage to be positioned or moved with high accuracy.